Method of fabricating back panel for liquid crystal display

ABSTRACT

In forming the back panel of a liquid crystal display, layers of an insulating material and aluminum are successively deposited on the surface of a semiconducting substrate having an array of electrodes thereon. Openings are then formed through the insulating material and the aluminum layer to expose the electrodes, thereby defining a spacer lattice which is integral with the substrate and whose walls are of a uniform height which corresponds to the desired spacing between the front and back panels of the liquid crystal display.

XE? 3 a 863 9 332 f W V r Unlted states 11 3,863,332

Leupp et al.- I Feb.'4, 1975 I METHOItOF FABRICATING BACK PANEL3,716,290 2/ 973 BOICI .Q 350/160 12c FOR LIQUID CRYSTAL DISpLAY3,756,924 9/1973 Collins 204/38 A 1 3,759,798 9/1973 Graff....'.. 204/58[75] Inventors: Alex'M. Leupp, Newport Beach;

. galn? G. DI, Costa Mesa, both of Primary Exammer Roy Lake a1 AssistantExaminer-W. C. Tupman [731- Assignee: Hughes Aircraft Company, Culver-Attorney, Agent, or Firm-W. l-I. MacAllister; E.

City, Calif.- 7 Szabo [22] Filed: June 28, 1973 1211 Appl. No.: 374,444-[57] ABSTRACT In forming the back panel of a liquid crystal display,layers of an insulating material and aluminum are suc- [52] 29/580350/160 56/17 cessively deposited on the surface ofa semiconducting I204/331 204/38 29/5921 5 substrate having an array of electrodesthereon. Open- ]IIL Cl..... B01] i gs are then formed through theinsulating material [58] Field of Search 29/578,580, 590, 591, thealuminum layer p086 the electrodes 592;:3550/160 204/33 A158; therebydefining a spacer lattice which is integral with v 156/17,, thesubstrate and whose walls are of a uniform height which corresponds tothe desired spacing between the [56] Rderences Cited v "front and backpanels of the liquid crystal display.

UNITED STATES PATENTS i w I 3,481,777 12/1969" Spannhake 29/625 13F'gures I5 25 l I l 1 I I I I *:1' v I a; 2| j \r 1 1 1 1 PATENTED FEB4|975 SHEU 10F 4 Fig. 2. I PRIOR ART Fig. 1. PRIOR ART PATENTEDFEB4.1915 3.863.332

SHEET 3!]? 4 PATENTEBFEB M975 3 86'3,332

SHEET UF 4 Fig. 12.

Fig. 13.

METHOD OF FABRICATING BACK PANEL FOR LIQUID CRYSTAL DISPLAY BACKGROUNDOF THE INVENTION The present invention relates generally to liquidcrystal displays and more particularly to a method for fabricating thebackplate for such displays having thereon spacers to maintain thethickness of the liquid crystal display uniform throughout.

In copending application Ser. No. 352,397 filed by Hans G. Dill, et al,on Apr. 18, 1973, and entitled Liquid Crystal Display System withIntegrated Signal Storage Circuitry, there is described a liquid crystaldisplay panel having a plurality of spacers between the back and frontpanels of the liquid crystal display. The purpose of these spacers is tomaintain auniform spacing between the front and back panels of theliquid crystal display. One of the features of the invention describedin the referenced application is the provision of addressing circuitrywhich is fabricated in the back panel of the display, which for thispurpose is a semiconducting wafer.

It is a principal object of the present invention to provide a methodfor the fabrication of spacers of the type disclosed in the referencedpatent application which method is compatible with the steps required tofabricate liquid display panels of the type therein described.

It is a related object of the invention to provide a method for thefabrication of back panels for liquid crystal displays with spacerswhich are accurately located with respect to an array of electrodes,also on the back panel.

A further object of the present invention is to provide a method forfabricating liquid crystal display back panels with integral spacersthereon wherein the spacers are resistant to attack by the liquidcrystal material, and may serve to shield electrically conductors thatrun to the electrodes.

In accordance with the invention the above and other objects areaccomplished by first forming an array of relective electrodes in spacedapart columns and rows on a surface of a substrate panel which ispreferably a semiconducting wafer. This step may be preceded by severalsteps directed to the formation of a plurality of switching devices inthe surface of the semiconducting substrate as described in thereferenced patent application. Following the formation of the reflectiveelectrodes there is deposited a layer of insulating material, preferablyoxide, on top of the substrate surface and over the electrodes, afterwhich an additional layer, preferably of aluminum, is deposited over thelayer of insulating material. A two-layered spacer lattice is formedfrom the successively deposited layers by removing those portions of thelayers which are over the central portions of the electrodes.Preferably, so much of the layers is removed as to expose all but theextreme perimeters of the array of electrodes, thereby providing aslight overlap of the two-layered lattice over the electrodes. Theresulting structure, .comprising the back panel having a plurality-ofelectrodes on its surface and a spacer lattice extending integrally fromthat surface, may then be used to complete the fabrication ofa liquidcrystal display by adding a front transparent panel having a transparentelectrode thereon and placing a nematic liquid crystal material betweenthe front and back. panels. Further objects and features of theinvention will become apparent from the following description anddrawings in which:

FIG. 1 is a perspective view of a liquid crystal display having a lessdesirable peripheral spacer between the front and back panels thereon;

FIG. 2 is a cross section through the display illustrated in FIG. 1;

FIG. 3 is a diagrammatic perspective view of a liquid crystal displayincorporating the spacer lattice configurationproduced in accordancewith the present invention;

FIG. 4 is a cross section through the display illustrated in FIG. 3',and

FIGS. 5-13 are a series of plan views and cross sections therethroughillustrating a back panel fabricated in accordancewith the presentinvention at successive stages of such fabrication. 1

Referring now to the figures, a liquid crystal display of conventionalconstruction is illustrated in FIGS. 1 and 2. A nematic liquid crystalmaterial 11 is confined between back and front plates 13 and 15 by aperipherally extending spacer 17. An array of electrodes 19 is disposedon the surface of the backplate l3 and a transparent common electrode(not shown) is disposed on the inside surface of the transparent frontplate 15. Desired images may be displayed by the selective actuation ofdesired ones of the array of electrodes 19 so as to establish anelectric field between them and the front electrode across the liquidcrystal material 11 lying between them. The particular theory ofoperation of liquid crystal displays is not of concern in thisapplication but may be gleaned from the above referenced applicationwhich is hereby incorporated by reference.

It is an inherent disadvantage of the peripheral spacer 17 that itpermits the bowing of the front electrode carrying plate 15, therebycausing uneven electric fields to be applied across the width and lengthof the display. As a result, different field strengths will exist acrossthe liquid crystal material'when various ones of the back electrodes 19are energized, causing uneven changes in the appearance of the liquidcrystal material across the display.

The above shortcomings are minimized by the provision of a spacerlattice in the liquid crystal display illustrated schematically in FIGS.3 and 4. The latter liquid crystal display is shown with the samecomponents as those illustrated in FIGS. 1 and 2 except that in place ofthe peripherally extending spacer 17 there is provided a lattice whosewalls crisscross the surface of the back panel 13 between the individualelectrodes 19. As best seen in FIG. 4, the individual walls of thespacer lattice 21 include a base 23 which rises above the surfaces ofthe electrodes 19'and a top portion 25 whose heights are uniformrelative to the surface of the backplate 13. Preferably, the bases 23are formed of an oxide layer which is particularly convenient to produceon top of the backplate 13 when that plate is made of silicon which is apreferable material for the backplate of the type disclosed in theabove-referenced patent application. Because of the limitations whichexist in the thickness to which such an oxide layer can be grown, it ispreferable that the top portions of the walls of the lattice 21 beformed of a different material and aluminum which is already used in theprocess of fabricating Turning now to FIGS. -13, there will be nextexplained a method for fabricating the spacer lattice of FIGS. 3 and 4in accordance with the present invention.

Prior to the fabrication of the spacer lattice, there is first formed ona suitable backplate 13 an array of electrodes 19. As described in theabove-referenced patent application, the backplate I3 is preferablyformed of a silicon substrate, typically in the form of a wafer abouttwo inches in diameter. Alternatively, of course, the wafer may besquare rather than circular in outline. In addition to forming an arrayof electrodes 19, arranged in columns and rows as shown in FIGS. 5 and6, there are also formed on the backplate 13 X and Y buslines orconductors 22 and 24. The purpose of the respective X bulines 22 is toconditionally enable all of the electrodes 19 in a row associated with aparticular busline. Similarly, it is the purpose of the respective Ybuslines 24 to conditionally enable all of the electrodes 19 in aparticular column associated with a particular y busline. Thus, when aparticular pair of X and Y buslines 22 and 24 is energized, this willcause a unique one of the electrodes 19 to be actuated. This isaccomplished by providing a switching transistor 26 for each electrode19. Since each of the transistors 26 functions as an AND gate, it isrepresented by the conventional symbol for such a gate in FIG. 5. Theswitching transistors 26 may be formed in the manner described in detailin the referenced patent application in the body of the siliconbackplate l3 and, since their fabrication is not a part of the presentinvention, it will not be described in detail herein. Briefly, however,the process described in the referenced patent application producesfield effect transistors in the surface of the backplate 13, each ofthese transistors having a source connected to its associated electrode19, a drain connected to one of the X and Y buslines 22 and 24, and agate connected to the other one of the buslines 22 and 24. Thus, byenergizing a particular pair of buslines 22 and 24, the necessaryconnections are made to the gate and drain of a particular transistor 26to complete a circuit through that transistor to the electrode 19 withwhich it is associated.

The transistors 26 are shown only schematically in FIG. 5 and are notshown physically either in that Figure or in FIG. 6, which is a crosssection therethrough. It will be understood, however, that the switchingtransistors 26 are physically located in the surface of the backplate13. Also disclosed in the referenced patent application is a techniquefor providing cross-under connections in the surface of the backplate 13for either the X or the Y buslines 22 and 24. Thus, assuming that it isthe X buslines 22 which are provided with such cross-under connections,a doped region is formed by conventional semiconductor doping techniquesin the surface of the substrate 13 under those regions of the Y buslines24 where they intersect 'the X buslines 22. Each of the X buslines 22makes contact with the doped cross-under connection on both sides of theY buslines to establish a continuous X bus conductor. Since thedeposition of the aluminum conductors and electrodes 22, 24 and 19 arepreceded by the formation of an oxide layer (not shown) in the processof forming the doped regions in the substrate which comprise thecross-under connectors, as well as the field effect transistors, thecross-under connections are prevented from directly connecting the Xconductors 22 to the Y conductors 24. Contact between the X conductors22 and their respective doped cross-under connections is established byetching through the oxide layer above these doped cross-underconnections so that when the metal layers, including the bus conductors22 are formed, they extend down to the cross-under connectors toestablish contact with them.

Returning now to the description of the present invention, after theformation of the display electrodes 19 and their associated buslines 22and 24, an insulating layer is deposited over the surface of thesubstrate 13 so as to cover both the surface and the electrodes 19.Preferably the insulating layer is a silicon dioxide film doped withphosphorus to effect a more uniform deposition. A thickness of 1.5microns can be readily achieved in a horizontal resistance heatedfurnace at 450C, maintained for 30 minutes. The doped silicon dioxidelayer is formed in the furnace by the decomposition of the SiH4 anddoping of the oxide can be achieved by adding phosphine gas Other meansof depositing the insulating layer, such as sputtering and evaporating,may also be employed.

There is next evaporated a layer of aluminum 27 on top of the dopedoxide layer 25.-As will become apparent shortly, it is the totalthickness of the two layers 25 and 27 which ultimately determines thetotal thickness of the liquid crystal display device in which thefabricated assembly will be incorporated. The optimum cell thickness andtherefore the optimum spacing between the back electrodes 19 and theelectrode carried by the front plate 15 depends on the particularapplication for which the display device is to be used. One of theadvantages of the present invention is that whatever is the desiredthickness it can be readily achieved by varying the thickness of thealuminum layer 27, since the evaporation process can be controlledwithin the required tolerances without difficulty. since front to backspacing of 6 to 10 microns is typical, the usual thickness of thealuminum layer will be about four to eight microns, which with the L5micron oxide layer 25 results in a total spacer height of5.5 to 9.5microns. In comparison a typical spacing between adjacent walls of thespacer will be about 10 mils, or about 25 times the height of the spacerwalls.

Windows 29 are next opened to the reflective back electrodes 19. Forthis purpose, a photoresist layer is applied on top of the aluminumlayer 27 and a suitable pattern in the photoresist layer is exposedafter which the photoresist is developed, the exposed portions areremoved, thereby exposing through openings 31 in the photoresist layerportions of the aluminum layer 27 corresponding to the windows 29 whichare to be formed therethrough. The masked aluminum layer 27 is thenexposed to an aluminum etchant which removes the exposed portions of thealuminum layer down to the bottom oxide-layer 25.

Next the remainder of the photoresist mask is removed and the remainingaluminum pattern 33 is anodized so as to minimize any possiblesubsequent interaction between the aluminum pattern 33 and the liquidcrystal material with which it will interface. Anodization is suitablyperformed electrolytically in a solution of tartaric acid (concentration3% Ph adjusted to 5.5 Application of volts between the aluminumstructure 33 and a negative electrode, both submerged in the tartaricacid solution, will result in an anodized layer of 2,100 angstroms.

Finally, using the anodized aluminum layer 35 as a mask, the exposedportions of the underlying oxide layer 25 are etched away in a solutionof buffered hydrofluoric acid. This etchant will expose the centralportion of each of the electrodes '19. In other words, the spacerlattice at its base overlaps the edges of the electrodes 19. v

This completes the fabrication'of the backplate and its integratedspacer lattice. Liquid crystal material may now be placed on thebackplate in the interstices of the spacer lattice, after which the topplate is secured in place.

What has been described in a-preferred method for fabricating anintegrated spacer lattice for the backplate of a liquid crystal cell;Modifications of the invention will readily occur to those skilled inthe art having the benefit of the above description. For example, othermaterials may be used in place of those given above for the two layers25 and 27. Moreover, a single layer of an insulating material which canbe grown to satisfy the heightrequirements for the spacer lattice can beused. -Such a material might, for example, be polycrystalline silicon.Moreover, whereas there have been shown an array of electrodes arrangedon the backplate of a liquid crystal cell in columns and rows, it isapparent that the method of the presentinvention for fabricating aspacer lattice structure for such a backplate could be employedwith-equal benefits with a different arrangement of such electrodes.Generally, it will be true that such a spacer lattice and methoddisclosed therefor'will be found useful wherever there are a pluralityof electrodes spaced from one anotheron the backplate of the liquidcrystal cell regardless of the geometrical configuration into which itsdistribution may fall.

We claim:

l. A method of fabricating a backplate with electrodes and integratedspacers for a liquid crystal display comprising the steps of:

a. forming an array of reflective electrodes in spaced apart columns androws on a surface of a substrate,

b. depositing an oxide layer on said surface and over said electrodes,

c. depositing an aluminum layer over said oxide layer, and

d. forming a two-layered lattice extending from the spaces between saidcolumnsand rows of electrodes by removing those portions of said layerswhich are over the central portions of said electrodes and anodizing thealuminum portions of said two-layered lattice prior to removing theportions of the oxide layer.

2. The method of claim 1 characterized further in that said step offorming a lattice includes the steps of:

a. forming a grid-shaped mask upon said aluminum layer, said maskcovering the aluminum layer above thespaces between said columns androws of electrodes and exposing the aluminum layer above at least thecentral portion of said electrodes,

b. etching away the exposed portions of said aluminum layer with anetchant which does not attack said oxide layer, thereby exposing thoseportions of said oxide layer which lie aboveat least the central portionof said electrodes,

c. anodizing the remaining portions of said aluminum layer, and

d. using said anodized aluminum portions as a mask, etching away theexposed portions of said oxide layer with an etchant which does notattack anodized aluminum.

3. A method of fabricating a backplate with electrodes and integratedspacers for a liquid crystal display comprising the steps of:

a. forming a plurality of electrodes on a surface of a substrate bydepositing aluminum upon said substrate and etching away portionsthereof,

b. forming a multi-layer on said substrate and over said electrodes bysuccessively forming an oxide layer and an aluminum layer on saidsubstrate, and

c. converting said multi-layer into a plurality of spaced apart walls ofequal height by successively etching through said aluminum layer andsaid oxide layer so as to expose said central portions of said pluralityof electrodes, said aluminum layer being anodized after it has beenetched but prior to the etching of said oxide layer, so as to make saidaluminum layer resistant to attack by nematic liquid crystal material.

1. A METHOD OF FABRICATING A BACKPLATE WITH ELECTRODES AND INTEGRATEDSPACERS FOR A LIQUID CRYSTAL DISPLAY COMPRISING THE STEPS OF: A. FORMINGAN ARRAY OF REFLECTIVE ELECTRODES IN SAPCED APART COLUMNS AND ROWS ON ASURFACE OF A SUBSTRATE, B. DEPOSITION AN OXIDE LAYER ON SAID SURFACE ANDOVER SAID ELECTRODES, C. DEPOSITING AN ALUMINIUM LAYER OVER SAID OXIDELAYER, AND D. FORMING A TWO-LAYERED LATTICE EXTENDING FROM THE SPACESBETWEEN SAID COLUMNS AND ROWS OF ELECTRODES BY REMOVING THOSE PORTIONSOF SAID LAYERS WHICH ARE OVER THE CENTRAL PORTIONS OF SAID ELECTRODESAND ANODIZING THE ALUMINIUM PORTIONS OF SAID TWO-LAYERED LATTICE PRIORTO REMOVING THE PORTIONS OF THE OXIDE LAYER.
 2. The method of claim 1characterized further in that said step of forming a lattice includesthe steps of: a. forming a grid-shaped mask upon said aluminum layer,said mask covering the aluminum layer above the spaces between saidcolumns and rows of electrodes and exposing the aluminum layer above atleast the central portion of said electrodes, b. etching away theexposed portions of said aluminum layer with an etchant which does notattack said oxide layer, thereby exposing those portions of said oxidelayer which lie above at least the central portion of said electrodes,c. anodizing the remaining portions of said aluminum layer, and d. usingsaid anodized aluminum portions as a mask, etching away the exposedportions of said oxide layer with an etchant which does not attackanodized aluminum.
 3. A method of fabricating a backplate withelectrodes and integrated spacers for a liquid crystal displaycomprising the steps of: a. forming a plurality of electrodes on asurface of a substrate by depositing aluminum upon said substrate andetching away portions thereof, b. forming a multi-layer on saidsubstrate and over said electrodes by successively forming an oxidelayer and an aluminum layer on said substrate, and c. converting saidmulti-layer into a plurality of spaced apart walls of equal height bysuccessively etching through said aluminum layer and said oxide layer soas to expose said central portions of said plurality of electrodes, saidaluminum layer being anodized after it has been etched but prior to theetching of said oxide layer, so as to make said aluminum layer resistantto attack by nematic liquid crystal material.